Class Sort

The fields used to determine sort order must be carefully chosen.
Documents must contain a single term in such a field,
and the value of the term should indicate the document's relative position in
a given sort order. The field must be indexed, but should not be tokenized,
and does not need to be stored (unless you happen to want it back with the
rest of your document data). In other words:

Valid Types of Values

There are four possible kinds of term values which may be put into
sorting fields: Integers, Longs, Floats, or Strings. Unless
SortField objects are specified, the type of value
in the field is determined by parsing the first term in the field.

Integer term values should contain only digits and an optional
preceding negative sign. Values must be base 10 and in the range
Integer.MIN_VALUE and Integer.MAX_VALUE inclusive.
Documents which should appear first in the sort
should have low value integers, later documents high values
(i.e. the documents should be numbered 1..n where
1 is the first and n the last).

Long term values should contain only digits and an optional
preceding negative sign. Values must be base 10 and in the range
Long.MIN_VALUE and Long.MAX_VALUE inclusive.
Documents which should appear first in the sort
should have low value integers, later documents high values.

Float term values should conform to values accepted by
Float.valueOf(String) (except that NaN
and Infinity are not supported).
Documents which should appear first in the sort
should have low values, later documents high values.

String term values can contain any valid String, but should
not be tokenized. The values are sorted according to their
natural order. Note that using this type
of term value has higher memory requirements than the other
two types.

Object Reuse

One of these objects can be
used multiple times and the sort order changed between usages.

This class is thread safe.

Memory Usage

Sorting uses of caches of term values maintained by the
internal HitQueue(s). The cache is static and contains an integer
or float array of length IndexReader.maxDoc() for each field
name for which a sort is performed. In other words, the size of the
cache in bytes is:

4 * IndexReader.maxDoc() * (# of different fields actually used to sort)

For String fields, the cache is larger: in addition to the
above array, the value of every term in the field is kept in memory.
If there are many unique terms in the field, this could
be quite large.

Note that the size of the cache is not affected by how many
fields are in the index and might be used to sort - only by
the ones actually used to sort a result set.